Field of the Invention
The present invention relates to a printing apparatus and a density correction method, specifically to a printing apparatus having functions of color shift correction and density unevenness correction, and a method thereof.
Description of the Related Art
An inkjet printing apparatus provided with print heads or nozzle arrays having ejection ports (hereinafter, referred to as nozzles) for the same color ink is conventionally known as one of output devices that prints an image on various printing media such as paper. In such a printing apparatus with print heads or nozzle arrays, due to difference in ejection characteristic among respective print heads or respective nozzle arrays, some printed images may not achieve a desired color tone. That is, a desired color tone cannot be obtained partly because of existence or occurrence of difference in ejection characteristic among respective print heads or respective nozzle arrays since difference in ejection characteristic changes a density value of a printed image.
Factors of such difference in ejection characteristic among respective print heads or respective nozzle arrays includes variations in heat generation amount of heaters that eject ink (or film thickness of heaters) and variations in diameter of nozzles that eject ink. A change in heat generation amount of a heater with age and a change in viscosity of ink due to difference in usage environment may cause difference in ink ejection amount, changing a printing characteristic of an image formed on a printing medium.
As a technology to deal with difference in color tone due to difference in ejection characteristic among respective nozzle arrays or print heads as described above, color shift correction processing is known.
For example, color shift correction processing is performed by changing a γ table that is used in γ correction processing as part of image processing for correcting an ejection characteristic of a print head. Specifically, patches are printed on a printing medium by print heads or nozzle arrays, and a table to be used in γ correction processing is reset to be suitable on the basis of the resulting printed patches.
A method of detecting a color shift in printed patches includes a method to use an input device such as a scanner to detect printed patches. For example, there is known a method of correcting color including: printing a patch for each color material of C, M, Y, K; reading each patch by a scanner, a colorimetric device, a densitometer and/or like that is mounted to a printing apparatus; detecting a deviation of a read value from an expectation value of each patch; and changing a value such as a γ value on the basis of the deviation (see Japanese Patent Laid-Open No. 2004-0.167947).
Meanwhile, there is known a method of adjusting a driving pulse width of a print head to be suitable by keeping constant a factor of a ratio of a real input voltage to a critical driving voltage at which a print head can barely eject ink due to deterioration of the print head (see Japanese Patent Laid-Open No. 2004-58529). In this method, a print head is driven while a pulse width is reduced by a predetermined unit, each ejected ink droplet is detected by a photoelectronic sensor, thereby deciding an optimal number of driving pulses. By changing to a driving pulse (hereinafter, referred to as an ejection energy) decided in this way, ink can be stably ejected, thereby lengthening a life of a print head.